Global Assessment of Multi-Mission Radar Altimeter Performance Over Land
James Garlick(1) and Philippa Berry(1)
De Montfort University,
Radar Altimeters have collected much useful data over the Earth's land surfaces during the past two decades. However, interpretation of the complex and rapidly varying echoes is problematic, especially when components are present from multiple facets within the pulse-limited footprint. One very effective way of analysing these data is to utilise a rule-based
expert system approach using a suite of retracking algorithms tailored for the multitude of echo shapes encountered.
The extent to which each altimeter can contribute to our knowledge of the Earth's land surfaces is dependent on both the orbit pattern and the onboard tracking used. ERS-1 and 2 sampled the majority of land in 'ice-mode' which increased the window size at the expense of vertical accuracy. This fact, particularly when combined with the ERS-1 Geodetic Mission orbit configuration has provided an invaluable legacy of land height data. The ENVISAT RA-2 further improved the acquisition of land data through the use of a mode-switching technique which dynamically adjusts the window size in order to maintain lock over more rapidly varying terrain whilst sampling moderate terrain at the highest available resolution. Despite being optimised for operation over the ocean, TOPEX, Jason-1, Geosat and GFO have also obtained significant amounts of data over land, particularly in flat to moderate terrain.
This paper presents an analysis of the performance of ERS-1, ERS-2, ENVISAT, TOPEX,
Jason-1, Geosat and GFO over land using a unique global multi-mission database of land heights obtained by retracking all echoes for which a leading edge is present. The extent to which each altimeter has contributed to the mapping of land heights is quantified. Particular improvements in acquisition caused by the tracker design are examined.
The results clearly demonstrate that all the altimeters included in this study have provided valuable height data over land, and have made a unique contribution to mapping of the Earth's topography.